1. Field of the Invention
This invention relates to the electronics industry, and, more specifically, to an apparatus and method for making quick, precise wire bonds between a micro-chip and a chip carrier.
2. Description of the Prior Art
In the past, semiconducter microprocessors and other micro-chips have been widely used. These micro devices provided powerful functions in tiny packages.
To enable such a chip to communicate with its environment, it was necessary to electrically inter-connect the chip to the surrounding circuitry. Initially, the electrical connections were effected by welding, or bonding, fine wires by hand with the aid of a microscope. The wires were connected to bonding pads on the chip and to leads on a chip carrier. The flexibility of a human operator allowed the bonds to made at any X-, Y- or Z-position, as well as in any theta angular orientation with respect to the surface of the chip. The travel in the X- and Y-directions defined the lateral position of the bond. The travel about the theta-axis aligned the bonding-wire feed axis with the desired path. The travel in the Z-direction primarily provided the contact between the bonding wire and the desired bonding site, but also provided sufficient motion to allow for variations in the elevations of the respective bonding pads. However, due to the diminutive character of the chip and the high degree of precision required, the bonding process was slow.
Subsequently, wire bonding machines were developed. These wire bonding machines utilized a chip holder and a bonding head. The chip holder typically held the chip with vacuum. The bonding head provided a bonding-wire feed and a transducer, which effected the actual bond. Wire-tear devices were incorporated into the bonding head to shear the bonding wire following the completion of a particular interconnection. Optical targeting systems were also mounted on the bonding head to precisely align the head with respect to the chip prior to bonding. Where significant height differences along the Z-axis existed between the various bonding sites, it was necessary to provide the optical targeting system with an additional focusing mechanism.
In some wire bonding machines, the chip holder moved in the X-, Y- and theta-directions, and the bonding head provided the Z-motion.
In other wire bonding machines, the chip holder moved in the theta-direction, and the bonding head provided the X-, Y- and Z-motions.
In still other wire bonding machines, the chip holder moved in the X-, Y-directions, while the bonding head provided the Z- and theta-motions.
In yet other wire bonding machines, the chip holder moved in only in the Z-direction, and the bonding head provided the X-, Y- and theta-motions.
U.S. Pat. No. 4,619,395 shows a workstation for holding a chip that provides both Z- and theta-motions for the chip. The disclosed device shows a chip holder pedestal rotatably mounted in a sliding support frame.
While the chip holders, or workstations, utilized in wire bonders of the past worked, a problem persisted. The chip holders of the past which did provide Z- and theta-motions for the chip did not affirmatively provide a directly coupled source of reference signals indicating the Z-position of the chip. A need existed for a wire bonder which eliminated Z- or theta-motions for the bonding head to thereby reduce the mass and interia of rotation of the bonding head and thereby allow an increased rate of operation, while affirmatively providing a directly coupled source of reference signals indicating the Z-position of the chip.